Remove orphaned metrics.py module

This module was no longer used after the decision to derive all
cost metrics from 100% trace capture. The backend aggregates costs
from traces by run_id, making standalone metrics redundant.

Signed-off-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Deborah Jacob <deborah@botanu.ai>

Author: deborahjacob-botanu